The invention relates to a discharge lamp provided with a tubular discharge vessel having an internal diameter of at most 5 mm, with a luminescent screen, and with a filling which comprises mercury and a rare gas.
Such a discharge lamp is known from EP 0562679 A1.
The rare gas used in the known discharge lamp usually consists mainly of argon. The known discharge lamp is highly suitable for use in a comparatively flat lighting unit on account of its small diameter. This increases the application possibilities of the discharge lamp considerably. Possible applications, for example, are the use of the discharge lamp in a lighting unit which serves as a backlight of an LCD screen or for the illumination of an instrument panel in an automobile. Other applications are in a lighting unit which forms a brake light or an indicator light of a vehicle. The flat shape of the lighting unit can be used in combination with widely differing shapes of the part of the vehicle on or in which the lighting unit is placed. A further advantage of such a discharge lamp is the comparatively high luminous efficacy (lm/W) during stationary lamp operation.
A major disadvantage of the known discharge lamp, however, is that the luminous flux of the discharge lamp immediately after ignition is comparatively low. This comparatively low luminous flux is caused by the fact that the quantity of mercury vapor present in the plasma immediately after ignition is considerably smaller than the quantity later during stationary lamp operation. It was found in practice that the initial luminous flux is lower in proportion as the internal diameter of the discharge vessel is smaller. The initial luminous flux of the lamp is also lower in proportion as the ambient temperature is lower. This comparatively low initial luminous flux renders the discharge lamp less suitable or even unsuitable for a large number of applications.